Product with a metallic basic body and method for manufacturing a product

ABSTRACT

A product and a method for manufacturing the product include a metallic basic body with at least one longitudinal duct disposed therein and with a number of transverse ducts branching off from the longitudinal duct. A covering layer lies on the outside of the basic body. The covering layer serves as a protective or adhesive layer. An enrichment layer covers walls of the longitudinal duct and of the transverse ducts and parts of the covering layer. A ceramic heat-insulating layer can also be provided on the outside.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of International ApplicationPCT/DE96/01207, filed Jul. 5, 1996, which designated the United States.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a product with a metallic basic body, at leastone longitudinal duct located inside the basic body and a number oftransverse ducts branching off from the longitudinal duct and eachhaving an associated outlet orifice in the basic body. The inventionrelates, moreover, to a method for manufacturing such a product. At thesame time, the invention relates particularly to such a product which isconstructed as a gas turbine component, especially as a blade.

In the case of stationary gas turbines (with previously conventionalmaterial temperatures of approximately 950° C.) and gas turbines inaircraft engines (with previously conventional inlet temperatures ofapproximately 1100° C.), an increase in inlet temperature has beenachieved by the use of specially developed alloys as basic materials forparts subjected to high thermal load, such as guide blades, movingblades, heat-shield elements and the like. Metal temperatures of wellabove 1000° C. can now be employed particularly as a result of the useof monocrystalline superalloys. The thermodynamic efficiency of a gasturbine can thereby be increased.

In addition to thermomechanical stresses, the components of gas turbinesare also exposed to chemical attacks, for example by flue gases attemperatures of up to and above 1300° C. In order to provide sufficientresistance to such attacks, such a component is covered with a metallicprotective layer. The protective layer must also have sufficiently goodmechanical properties. Particularly in view of the mechanicalinteraction between the protective layer and the basic material of thecomponent, the protective layer should be sufficiently ductile to becapable of matching possible deformations of the basic material. Itshould also be as unsusceptible to cracking as possible, as a preventionfrom being laid bare, along with subsequent corrosion and oxidation ofthe basic material.

Metallic protective layers for metallic components, especially forcomponents of gas turbines, which are used for increasing resistance tocorrosion and/or to oxidation, are known in a wide diversity in theprior art. One class of alloys for protective layers is known by thecollective term "MCrAlY alloys", with M standing for at least one of theelements from the group including iron (Fe), cobalt (Co) and nickel(Ni), and further essential constituents being chromium (Cr), aluminum(Al) and yttrium (Y).

A protective layer composed of an MCrAlY alloy, which improves thecorrosion and oxidation properties of a product within a surfacetemperature range of 600 to 1150° C., is described in Published EuropeanPatent Application 0 412 397 A1. The protective layer has a fraction of1-20% rhenium in addition to 22-60% chromium, 0-15% aluminum, 0.3-2%yttrium or 0.3-2% of another element from the rare-earth group. Thebasis of the alloy is nickel and if appropriate, further elements may beadded, especially cobalt. Due to the good thermal conductivity of themetallic protective layer, the component covered with the protectivelayer is exposed to virtually the same thermal load as the protectivelayer itself.

A further corrosion-resistant protective coating for components of gasturbines and further components formed of nickel-based or cobalt-basedalloys is known from European Patent 0 486 489 B1. That protectivecoating contains the following elements (given in parts by weight):25-40% nickel, 28-32% chromium, 7-9% aluminum, 1-2% silicon, at least 5%cobalt, and 0.3-1% rare earths, especially yttrium. The properties ofthe individual constituents are specified explicitly in thatpublication.

A two-ply metallic protective layer composed of two different alloys isdescribed in European Patent 0 397 731 B1, corresponding to U.S. Pat.No. 5,499,905. The outer alloy is an MCrAlY alloy and contains (given inparts by weight) 15-40% chromium, 3-15% aluminum and 0.2-3% of at leastone element from the group including yttrium, tantalum, hafnium,scandium, zirconium, niobium and silicon. That outer alloy is itselfcovered with a thermal barrier layer for protection against particularlyhigh temperatures, if appropriate, particularly in the case ofinternally cooled metal articles. The thermal barrier layer can bezirconium oxide with an addition of yttrium oxide. Oxidation of theouter alloy before the application of the thermal barrier layer isprovided in order to prevent the thermal barrier layer from possiblyflaking off from the outer alloy.

In the prior art, it is also known, in the case of a turbine blade, tocarry out an internal coating of the relatively narrow cooling ductswith a metal, for example with aluminum (see the paper by J. E. Restallet al., entitled "A Process for Protecting Gas Turbine Blade CoolingPassages Against Degradation", Superalloys, 1980, pp. 405-410). Afurther method for depositing aluminum on a nickel compound, whichmethod can also be used for inner surfaces and cooling ducts, is alsodescribed in the literature (a paper by R. S. Parzuchowski: entitled"Gas Phase Deposition of Aluminum of Nickel Alloys", in Thin Solid Films45, 1977, pp. 349-355). The use of chromium or of a combination ofaluminum and chromium is also possible. Reference must additionally bemade to German Patent DE 41 19 967 C1. It may be stated that the priorart generally only knows of internal coatings for cooling ducts togetherwith identical external coatings.

Blades for highly developed gas turbines, for example for aircraftengines, and increasingly for stationary gas turbines as well, arenowadays of complex construction. A distinction can, in that case, bemade between the following features: a metallic basic body, that is tosay the actual blade, is cast hollow and thin-walled from ahigh-temperature material. Efficient cooling through the use of acooling medium, especially a gas, such as air, from the inside of theblade is thereby to become possible. For that purpose the basic body hasat least one longitudinal cooling duct and a number of transversecooling ducts branching off therefrom.

A coating which protects the metallic basic body against oxidation andhigh-temperature corrosion is provided on the hot-gas side of the blade.In many instances, there is, on the coating, a further coating locatedon the hot gas side and formed of a ceramic material, for the purpose ofreducing the heat flux in the blade. An internal coating is alsodesirable for protection against an oxidation-related weakening of thewall thickness and the initiation of cracking on the coolant side. Inthat case, the transverse cooling ducts may be considered asperforations in the blade leaf and/or the platform or platforms, withthe cooling medium emerging through those perforations. Particularlygood distribution and, where appropriate, the formation of acooling-medium mist on the hot-gas side as well can be achieved thereby.That mist leads to film cooling.

SUMMARY OF THE INVENTION

It is accordingly an object of the invention to provide a product with ametallic basic body and a method for manufacturing such a product, whichovercome the hereinafore-mentioned disadvantages of the heretofore-knownproducts and methods of this general type, in which the product iscapable of being manufactured cost-effectively and having an externaland an internal coating, and in which the manufacturing method iscost-effective and, in particular, provides all transverse ducts with acoating, without the cross-section thereof being narrowed in anuncontrolled manner.

With the foregoing and other objects in view there is provided, inaccordance with the invention, a product, comprising a metallic basicbody formed of an alloy and having an outside; the basic body having atleast one longitudinal duct formed therein, a number of transverse ductsbranching off from the longitudinal duct in the basic body, and outletorifices each associated with a respective one of the transverse ductsin the basic body; a metallic covering layer applied directly onto theoutside of the basic body and having an alloy different from the alloyof the basic body; and a metallic enrichment layer covering the basicbody only in the longitudinal duct and in the transverse ducts, forminga coated longitudinal cooling duct and a number of coated transversecooling ducts branching off from the coated longitudinal cooling duct,for passage of a cooling medium flow through the coated longitudinal andtransverse cooling ducts, and the enrichment layer additionally coveringa small part of the covering layer at each of the outlet orifices.

In accordance with another feature of the invention, the covering layeris formed of an MCrAlY alloy and, furthermore, has a thickness of 180 μmto 300 μm. In particular, the alloys known from Published EuropeanPatent Application 0 412 397 A1 and European Patent 0 486 489 B1 comeinto consideration as an MCrAlY alloy.

In accordance with a further feature of the invention, the enrichmentlayer on the product has a thickness of 30 μm to 100 μm.

In accordance with an added feature of the invention, the enrichmentlayer is constructed as a diffusion layer, that is to say as a layerwhich is formed in the basic body by diffusing-in a separately appliedmetal.

In accordance with an additional feature of the invention, in particularaluminum, chromium and chromium/aluminum alloys come under considerationas such a metal, with aluminum without chromium being especiallypreferred.

In accordance with yet another feature of the invention, there isprovided a ceramic heat-insulating layer which covers the outside of thecovering layer and, at each outlet orifice, also the enrichment layer onthe small parts of the covering layer where the enrichment layer coversthe covering layer.

In accordance with yet a further feature of the invention, theheat-insulating layer has a thickness of 100 μm to 500 μm, especially of200 μm to 300 μm.

In accordance with yet an added feature of the invention, the product,particularly with one or more of the above-described preferreddevelopments, is constructed as a gas turbine component, for example asa blade or a heat-shield element. Its actual features make itparticularly apt to interpret it as being capable of withstanding themechanical, thermal and chemical loads which are to be expected duringoperation in a gas turbine, in which case hot flue gas flows round theproduct.

With the objects of the invention in view, there is also provided afirst version of a method for manufacturing a product, which comprisesthe following sequence of steps applying a metallic covering layer on abasic body having at least one longitudinal duct inside the basic body;boring or drilling transverse ducts through the basic body and thecovering layer to the longitudinal duct and forming outlet orifices ofthe transverse ducts; applying an enrichment layer to the basic body inthe longitudinal duct, in the transverse ducts and in each case on smallparts of the covering layer at the outlet orifices; heat treating thebasic body with the covering layer and with the enrichment layer; andsmoothing the covering layer.

With the objects of the invention in view, there is additionallyprovided a second version of a method for manufacturing a product, whichcomprises the following sequence of steps applying a metallic coveringlayer on a basic body having at least one longitudinal duct inside thebasic body; boring or drilling transverse ducts through the basic bodyand the covering layer to the longitudinal duct and forming outletorifices of the transverse ducts; applying an enrichment layer to thebasic body in the longitudinal duct, in the transverse ducts and in eachcase on small parts of the covering layer at the outlet orifices on thecovering layer; smoothing the covering layer; applying a ceramicheat-insulating layer on the covering layer; and heat treating the basicbody with the covering layer, with the enrichment layer and with theceramic heat-insulating layer.

With regard to the first version of the method, it is noted that thesmoothing of the covering layer serves particularly for removing asurface layer which has occurred at undesirable locations during theapplication of the enrichment layer and which is enriched with thematerial used for forming the enrichment layer. Regarding the secondversion of the method according to the invention, it is noted that thestep of smoothing the covering layer is carried out according to therequirements of the ceramic heat-insulating layer to be applied, whereinan undesirable surface layer which has possibly occurred on the coveringlayer is once again removed.

Within the scope of the first version of the method, the covering layerconstitutes, in particular, a protective layer which is to protect thebasic body against corrosion and/or oxidation. Within the scope of thesecond version of the method, the covering layer serves particularly asan adhesive layer, in order to tie the ceramic heat-insulating layer tothe basic body. This tying possibly takes place through a thin oxidicfilm occurring on the covering layer. This film may occur as a result ofthe oxidation of the covering layer and it may also be applied in aseparate step. If appropriate, a film formed by the oxidation of thecovering layer may also be modified before the application of theceramic heat-insulating layer, in particular by the introduction of afurther chemical element, such as, for example, nitrogen.

In accordance with another mode of the invention, within the scope ofeither version of the method, the covering layer can be applied throughthe use of a low-pressure plasma spraying method (LPPS) or a vacuumplasma spraying method (VPS). The vacuum plasma spraying methodparticularly is preferred for applying a covering layer composed of anMCrAlY alloy.

In accordance with a further mode of the invention, in order to applythe enrichment layer, at least one of the elements aluminum andchromium, and preferably aluminum, is vapor-deposited onto the basicbody and diffused-in, so that the enrichment layer forms by the additionof aluminum or chromium to the material of the basic body or thecovering layer.

In accordance with an added mode of the invention, the drilling of thetransverse ducts in the basic body is carried out by a laser drillingmethod, an electrochemical countersinking method (ECM) or an electricaldischarge method (EDM).

In accordance with an additional mode of the invention, if aheat-insulating layer is to be applied within the scope of the method,it is carried out through the use of an atmospheric plasma sprayingmethod (APS) or a physical vapor deposition method (PVD). In this case,the plasma spraying method provides an essentially unstructured ceramicheat-insulating layer in an especially cost-effective way, whereas thevapor deposition method, which as a rule is more expensive than thespraying method, can provide a ceramic heat-insulating layer which isformed of individual columnar crystallites grown in each case on thecovering layer. In comparison with an unstructured heat-insulatinglayer, such a columnar-crystalline heat-insulating layer has significantadvantages. However, such advantages have to be acquired at the expenseof markedly higher manufacturing costs. The choice between anunstructured and a columnar-crystalline heat-insulating layer musttherefore be decided specially for each individual case.

In accordance with a concomitant mode of the invention, the heattreatment provided within the scope of each version of the method servesfor solution annealing and/or hardening of the coated basic body.

A particular advantage of the invention is that the outer surface doesnot have to be covered while the internal coating is being applied.Moreover, the order of manufacture and the work steps, especiallysmoothing, which follow coating ensure that phases with an increasedcontent of material of the enrichment layer, especially aluminum, do notoccur or remain either between the component surface and the coveringlayer or on the covering layer. In particular, such phases are known totend to form cracks. The formation of cracks can therefore largely beavoided.

The method according to the invention guarantees, moreover, that all ofthe transverse cooling ducts, that is to say all of the cooling-airoutlet bores, are coated.

The enrichment layer is preferably applied by a CVD method (ChemicalVapor Deposition), in particular a diffusion process. This choice of themethod for applying the internal coating keeps contamination of theouter surface low. Moreover, since the latter is formed of the stillspray-rough covering layer which is preferably produced by the VPS(Vacuum Pressure Spraying) method or by the LPPS (Low Pressure PlasmaSpraying) method, a complete removal of all undesirable residues can beachieved in the subsequent smoothing operation, which necessarily has tobe an abrasive method (grinding process). Furthermore, the number ofheat treatments can remain relatively small.

If the component is to acquire a heat-insulating layer, this mustpreferably be applied through the use of a PVD method (Physical VaporDeposition).

The products described above each have a relatively long life as turbinecomponents.

In terms of the order of manufacture, the advantage of the manufacturingmethod is that the transverse ducts, that is to say the cooling-airbores, are not closed, but are only narrowed in a closely reproduciblemanner. This can be demonstrated in the construction of a component tothe size of the drawing.

Other features which are considered as characteristic for the inventionare set forth in the appended claims.

Although the invention is illustrated and described herein as embodiedin a product with a metallic basic body and a method for manufacturing aproduct, it is nevertheless not intended to be limited to the detailsshown, since various modifications and structural changes may be madetherein without departing from the spirit of the invention and withinthe scope and range of equivalents of the claims.

The construction and method of operation of the invention, however,together with additional objects and advantages thereof will be bestunderstood from the following description of specific embodiments whenread in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary, diagrammatic, sectional view of a gas turbineblade without an outer heat-insulating layer;

FIG. 2 is a view similar to FIG. 1 of a gas turbine blade with an outerheat-insulating layer;

FIG. 3 is a flow diagram for the manufacture of a gas turbine bladeaccording to FIG. 1; and

FIG. 4 is a flow diagram for the manufacture of a gas turbine bladeaccording to FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to the figures of the drawings in detail and first,particularly, to FIG. 1 thereof, there is seen a blade 2 for a gasturbine having a metallic basic body 4. The basic body 4 can, inparticular, be one which is formed of a nickel-based or cobalt-basedsuperalloy. A longitudinal duct 6 is located approximately centrallyinside the basic body 4. A number of transverse ducts 8 branch off fromthis longitudinal duct 6. As will become clear below, after internalcoating, the longitudinal duct 6 and the transverse ducts 8 serve forthe conduction of a cooling medium A, especially a cooling gas, such asair.

In each case a covering layer 10 is applied directly externally on eachside of the basic body 4. This covering layer 10 is formed of an MCrAlYalloy and it preferably has a thickness of 180 μm to 300 μm. Outletorifices 14 are at the same time left free. The covering layer 10 ispreferably applied through the use of a low-pressure plasma or vacuumplasma spraying method in each case and it performs the function of an(outer) protective layer.

In order to provide an internal coating, an enrichment layer 12 isprovided. This layer covers not only walls of the longitudinal duct 6and walls of the transverse ducts 8. On the contrary, it is also locatedin an outer region of the transverse ducts 8, leaving the outletorifices 14 free, and at the same time it laterally covers a small partof the covering layer 10. This covered part is designated by referencesymbol 16. The enrichment layer 12 preferably has a thickness of 30 μmto 100 μm. The enrichment layer is preferably applied through the use ofa diffusion method, in which chromium and/or aluminum is vapor-depositedand diffused in.

It is evident that the blade 2 thus has a coated longitudinal coolingduct 6a and a number of coated transverse cooling ducts 8a branching offfrom the latter, for the cooling medium A to flow through.

The blade 2 of FIG. 2 largely corresponds to that of FIG. 1. In thiscase, however, a ceramic heat-insulating layer 20 is also provided onthe outside, that is to say on the covering layer 10. The covering layer10, which once again preferably is formed of MCrAlY, serves the functionof an adhesive layer in this case. The heat-insulating layer 20 has athickness of 100 μm to 500 μm and preferably a thickness of 200 μm to300 μm and it can be formed of one of the conventional known materials.It is noteworthy that the heat-insulating layer 20 covers the coveringlayer 10 on the outside and an outer region of the transverse ducts 8,leaving the outlet orifices 14 free, as well as a small part or overlapregion 22 of the enrichment layer 12. The heat-insulating layer 20 canbe applied through the use of an atmospheric plasma spraying method(APS) or through the use of a physical vapor-deposition method (PVD).

FIG. 3 shows a basic procedure for manufacturing a blade 2 according toFIG. 1. According to FIG. 3, initially casting, that is to say themanufacture of the shaped basic body 4 including the longitudinal duct6, takes place in a first step 30. A plurality of longitudinal ducts 6can also be provided. Mechanical machining is carried out in a secondstep 32. In this case, milling of a blade root, milling of sealingsurfaces of the blade 4 and/or another machining step are carried out,so that a blank is obtained. In the next step 34, an application of thecovering layer 10 on the basic body 4 takes place. This covering layer10 can be formed particularly of an MCrAlY alloy. The application iscarried out through the use of a low-pressure or vacuum plasma sprayingmethod (Low Pressure Plasma Spraying=LPPS or Vacuum PlasmaSpraying=VPS). At the same time, if appropriate, the blank is subjectedto tying heat treatment. The covering layer 10 serves as a protectivelayer while the blade 2 is in operation.

In the next step 36, drilling or boring of the transverse ducts 8 iscarried out. Various technologies can be utilized for this purpose. Ifthey are ducts 8 with a round cross-section and feed conduits to shapedoutlet orifices, laser machining can be carried out. In contrast, ifthey are film-cooling bores which are, for example, trapezoidal orotherwise shaped in cross-section, an electrochemical countersinkingmethod (Electro Chemical Milling=ECM) or electrical dischargecountersinking (Electrical Discharge Milling=EDM) can be employed. Thisis followed by step 38, specifically internal coating. This involves theapplication of the enrichment layer 12. This application can be carriedout, for example, through the use of a reactive gas according to adiffusion process (Chemical Vapor Deposition=CVD) or according to apowder packing method with a subsequent diffusion process. It wasalready pointed out at the outset that such methods are known per se.

After the basic body 4 has therefore acquired its metallic external andinternal coating 10, 12, it is delivered for heat treatment in a step40. This step 40 is necessary so that the material of the basic body 4acquires its optimum material properties. This step 40 involves, inparticular, solution annealing and subsequent hardening. In the nextstep 42, the roughness of the then finished blade 4 is eliminated. Thisis carried out through the use of the mechanical process of smoothing.In this case, residues on the surface of the covering layer 10 are alsoremoved, as a result of which, for example, the initiation of crackingby brittle aluminum-rich phases is avoided.

In FIG. 4, steps 30 to 38 correspond to steps 30 to 38 of FIG. 3. Arepeated description is therefore dispensed with.

In FIG. 4, step 38 is followed by a step 44 of mechanical smoothing. Inthis case, the surface is prepared for the following application of theheat-insulating layer 20 in a step 46.

In the step 46, the application of the heat-insulating layer 20 iscarried out, specifically through the use of a vapor-deposition process.In order to provide for this, the electron-beam vapor-deposition process(Electron Beam Physical Vapor Deposition=EB-PVD) is preferably employed.Whereas the blade 2, as manufactured according to FIG. 3, has a metallicsurface towards the outside, the blade 2 according to FIG. 4 has aceramic surface towards the outside.

Step 46 is followed by a step 48 for heat treatment (according to step40 of FIG. 3). In this case too, this involves a solution annealing andcuring of the basic material of the blade 2. After this step 48, theblade 2 according to FIG. 2 is available for use.

We claim:
 1. A product, comprising:a) a metallic basic body formed of analloy and having an outside; b) said basic body having at least onelongitudinal duct formed therein, a number of transverse ducts branchingoff from said longitudinal duct in said basic body, and outlet orificeseach associated with a respective one of said transverse ducts in saidbasic body; c) a metallic covering layer applied directly onto saidoutside of said basic body and having an alloy different from said alloyof said basic body; and d) a metallic enrichment layer covering saidbasic body only in said longitudinal duct and in said transverse ducts,forming a coated longitudinal cooling duct and a number of coatedtransverse cooling ducts branching off from said coated longitudinalcooling duct, for passage of a cooling medium flow through said coatedlongitudinal and transverse cooling ducts, and said enrichment layeradditionally covering a small part of said covering layer at each ofsaid outlet orifices, leaving said metallic covering layer substantiallyuncovered.
 2. The product according to claim 1, wherein said coveringlayer is formed of an MCrAlY alloy, where M is an element selected fromthe group consisting of iron, cobalt, and nickel.
 3. The productaccording to claim 2, wherein said covering layer has a thickness of 180μm to 300 μm.
 4. The product according to claim 1, wherein saidenrichment layer has a thickness of 30 μm to 100 μm.
 5. The productaccording to claim 1, wherein said enrichment layer is a diffusionlayer.
 6. The product according to claim 1, wherein said enrichmentlayer contains at least one of aluminum and chromium as an essentialconstituent.
 7. The product according to claim 1, wherein saidenrichment layer contains only aluminum as an essential constituent. 8.The product according to claim 1, including a ceramic heat-insulatinglayer covering the outside of said covering layer and said enrichmentlayer on said small part of said covering layer at each of said outletorifices.
 9. The product according to claim 8, wherein saidheat-insulating layer has a thickness of 100 μm to 500 μm.
 10. Theproduct according to claim 8, wherein said heat-insulating layer has athickness of 200 μm to 300 μm.
 11. A gas turbine component,comprising:a) a metallic basic component body formed of an alloy andhaving an outside; b) said basic component body having at least onelongitudinal duct formed therein, a number of transverse ducts branchingoff from said longitudinal duct in said basic component body, and outletorifices each associated with a respective one of said transverse ductsin said basic component body; c) a metallic covering layer applieddirectly onto said outside of said basic component body and having analloy different from said alloy of said basic component body; and d) ametallic enrichment layer covering said basic component body only insaid longitudinal duct and in said transverse ducts, forming a coatedlongitudinal cooling duct and a number of coated transverse coolingducts branching off from said coated longitudinal cooling duct, forpassage of a cooling medium flow through said coated longitudinal andtransverse cooling ducts, and said enrichment layer additionallycovering a small part of said covering layer at each of said outletorifices, leaving said metallic covering layer substantially uncovered.12. A gas turbine blade, comprising:a) a metallic basic blade bodyformed of an alloy and having an outside; b) said basic blade bodyhaving at least one longitudinal duct formed therein, a number oftransverse ducts branching off from said longitudinal duct in said basicblade body, and outlet orifices each associated with a respective one ofsaid transverse ducts in said basic blade body; c) a metallic coveringlayer applied directly onto said outside of said basic blade body andhaving an alloy different from said alloy of said basic blade body; andd) a metallic enrichment layer covering said basic blade body only insaid longitudinal duct and in said transverse ducts, forming a coatedlongitudinal cooling duct and a number of coated transverse coolingducts branching off from said coated longitudinal cooling duct, forpassage of a cooling medium flow through said coated longitudinal andtransverse cooling ducts, and said enrichment layer additionallycovering a small part of said covering layer at each of said outletorifices, leaving said metallic covering layer substantially uncovered.